1. Field of the Invention
The present invention relates to a liquid crystal display(LCD) device and, more particularly, to a flexible liquid crystal display device using a flexible plate as a substrate.
2. Description of the Prior Art
LCD device displays an image using light irradiated from a back light, then projected through a polarizing layer by an optical switching effect of a liquid crystal layer, or by using an external light reflected on the reflection layer by switching effect of the liquid crystal layer.
Nowadays, some LCD devices are manufactured such that substrates are made of a flexible plate, such as a plastic film, so that the devices can be mounted even on a non-planar surface.
For example, such an LCD device using a plastic film integrated with a polarizing layer as a substrate is disclosed in U.S. Pat. No. 4,674,840 to Bennett. The LCD device of Bennett includes a pair of substrate assemblies opposing to each other. Each of the substrate assemblies includes a flexible substrate made of, for example, polyethylene terephthalate, and a polarizing layer laminated on the substrate by means of an intermediate adhesive. Furthermore, a protective layer, a conductive layer, and an orientation layer are, in this order, deposited on the inner surface of the polarizing layer. A layer of liquid crystal material is disposed between the two substrate assemblies spaced from one another with a substantially parallel relationship. The conductive layer is positioned directly on the polarizing layer. One of the substrate assemblies optionally has a reflection layer positioned intermediate a polymeric support and the polarizing layer.
Such an LCD using the plastic film integrated with the polarizing layer as a substrate has a drawback in that the polarizing layer is weak against heat, causing problems during a high temperature manufacturing process.
U.S. Pat. No. 4,674,840 discloses a stretched polyvinyl alcohol as a suitable material to serve as the polarizing layer. However, since the polyvinyl alcohol has a melting point at 90.degree. C., it cannot endure at an orientation layer forming temperature which is in the range of 200-240.degree. C. Therefore, the polarizing layer have to be attached using an adhesive after a heat treatment process for forming the orientation layer is finished. As another material suitable for the polarizing layer, iodide is well-known. However, when using the iodide as the polarizing layer, it is impossible to bond the iodide to plastic substrate because the melting point of the iodide is at 80-90.degree. C.
Furthermore, the plastic film on which the polarizing layer is laminated cannot be applied to an STN LCD since the STN LCD should be provided with a retardation film on the polarizing layer.
On the other hand, an LCD device having a structure in which a retardation film and a polarizing layer are laminated on a plastic film can be considered. However a substrate consisting of many sheets has a drawback in that the transmittance of a back light and the reflection light degrade. Also, because one of the substrate assemblies needs to have a non-directional characteristic and a birefringence index less than 15 nm, a special material should be used for this substrate assemblies, increasing manufacturing costs.